A Nato force including up to 3,000 British troops will be deployed to Iraq to support the vulnerable new government as it takes over the running of the country, under a plan being drawn up in London and Washington.

The force would consist of Nato's Allied Rapid Reaction Corps, based in Germany under the command of a British general, Sir Richard Dannatt, reinforced by a British battle group.

Officials said yesterday the corps would be "temporarily extracted" from Nato to make it more politically acceptable to members of the alliance - notably France and Germany - that were opposed to the war, as well as Russia and many Iraqi leaders.

The new force would not be deployed under a Nato banner but would be described as a British-led international force. About 60% of the corps' staff are British.

Officials said the plan is expected to be formally agreed at the Nato summit in Istanbul on the eve of the official handover of sovereignty on June 30 to the interim Iraqi government led by Ayad Allawi.

Britain's chiefs of staff have been working on plans for weeks involving the deployment of up to 3,000 troops to south central Iraq, including the holy city of Najaf, the scene of fighting between US forces and the militia of the radical cleric Moqtada al-Sadr.

Defence sources said the plan would increase pressure on the army but that it could live with it for a finite period. A deployment of 18 months is envisaged.

The government announced on Thursday that 600 Royal Marines are to be deployed to bolster British forces in southern Iraq. Though they will replace an army battalion, the deployment will increase the number of British troops in Iraq by around 270, to 9,200.

The proposed British-led international force is likely to include troops from several members of the alliance.

The US has been struggling to find countries willing to put up troops for Iraq, even after the UN resolution agreed earlier this month on the status of the interim government.

British military comman ders have insisted that the role of their troops must be seen in the overall strategic, political, and economic situation in Iraq. Their concerns were reflected in a leaked Foreign Office memo last month. In the event of Britain agreeing to send reinforcements, it said, "we must ensure that ... we can prevent US action, either at the strategic or operational levels, which would jeopardise our objectives".

It added: "We need a more flexible approach towards cooperation with existing militias". Mr Sadr's decision this week, under pressure from moderate Shia leaders and perhaps with political ambitions in mind, to order his Mehdi militia to go home, should make the task of a new international force easier.

The US-led coalition force in Iraq received a much-needed boost yesterday when South Korea announced it is to deploy more than 3,000 troops. They will be based in Arbil, in Iraqi Kurdistan, a relatively peaceful region.

The original plan in April was to base them in Kirkuk, an area disputed between Iraqi Kurds and Iraqi Arabs and the scene of several bombings. But the South Korea government, under pressure from domestic opposition to sending troops to Iraq, insists that it can only participate in peace-keeping rather than take a combat role.

The first of the troops, about 900, are to be deployed in early August, with another 1,000 the following month, and another 1,000 later. South Korea already has about 670 military engineers and medical staff at Nassiriya, in southern Iraq, and they will join the larger deployment at Arbil. About half the troops are combat-ready forces.

Meanwhile, Jaap de Hoop Scheffer, Nato's secretary general, said yesterday the Istanbul summit would also decide on "a stronger Nato presence" in Afghanistan.

There are 20,000 foreign, mainly US, troops in Afghanistan in addition to the Canadian-led 6,400-strong International Security Assistance Force (Isaf), whose troops come from Nato and other countries and are concentrated in Kabul.